Prior art FIG. 1 illustrates a conventional flip chip LED die 10 mounted on a portion of a submount wafer 12. In a flip-chip, both the n and p contacts are formed on the same side of the LED die.
The LED die 10 is formed of semiconductor epitaxial layers, including an n-layer 14, an active layer 15, and a p-layer 16, grown on a growth substrate, such as a sapphire substrate. The growth substrate has been removed in FIG. 1 by laser lift-off, etching, grinding, or by other techniques. In one example, the epitaxial layers are GaN based, and the active layer 15 emits blue light. LED dies that emit UV light are also applicable to the present invention.
A metal electrode 18 electrically contacts the p-layer 16, and a metal electrode 20 electrically contacts the n-layer 14. In one example, the electrodes 18 and 20 are gold pads that are ultrasonically welded to anode and cathode metal pads 22 and 24 on a ceramic submount wafer 12. The submount wafer 12 has conductive vias 24 leading to bottom metal pads 26 and 28 for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer 12 and will be later singulated to form individual LEDs/submounts.
Further details of LEDs can be found in the assignee's U.S. Pat. Nos. 6,649,440 and 6,274,399, and U.S. Patent Publications US 2006/0281203 A1 and 2005/0269582 A1, all incorporated herein by reference.
While an array of LED dies 10 are mounted on the submount wafer 12 or after the wafer 12 is diced, it is well known to deposit a phosphor over each LED die to generate any desired light color. To produce white light using the blue LED die 10, it is well known to deposit a YAG phosphor, or red and green phosphors, directly over the die 10 by, for example, spraying or spin-coating the phosphor in a binder, electrophoresis, applying the phosphor in a reflective cup, or other means. It is also known to affix a preformed tile of phosphor (e.g., a sintered phosphor powder or phosphor powder in a binder) on the top of the LED die 10. Blue light leaking through the phosphor, combined with the phosphor light, produces white light. Problems with creating the phosphor layer over the LED die 10 include the difficulty in creating very uniform phosphor layer thicknesses and densities. Any variation in the thickness or density will result in color non-uniformity over the surface of the LED die. A preformed tile of phosphor may be made more uniform and allows color testing of the tile prior to affixing it to the LED die; however, it is difficult and time-consuming to precisely affix each tile (e.g., 1 mm2) to the top surface of an LED die 10.
Additionally, if a phosphor layer is deposited over all the LED dies 10 while the LED dies 10 are mounted on the submount wafer 12, prior to dicing the wafer 12, much of the phosphor will be wasted since it would be deposited on portions of the wafer 12 in-between the LED dies 10.
What is needed is a technique to create a phosphor layer over an LED die that does not suffer from the drawbacks of the prior art.